1. Field of the Invention
This invention relates to a novel process for preparation of tetraalkylthiuram disulfides. More particularly, it relates to the process for preparing the disulfides characterized in that an aqueous solution containing an alkali metal dialkyldithiocarbamate is subjected to electrolytic oxidation in the presence or absence of a supporting electrolyte, which aqueous layer is placed over a layer consisting essentially of carbon disulfide or a mixed solvent of carbon disulfide as a predominant component and a halogenated alkane having a higher density than water, to obtain directly the corresponding tetraalkylthiuram disulfide.
2. Description of the Prior Art
A typical industrial process for production of tetraalkylthiuram disulfides to be used as vulcanizing accelerators or agents is as follows. An aqueous solution of a sodium dialkyldithiocarbamate is prepared by reacting the corresponding dialkylamine and carbon disulfide at a low temperature in the presence of an aqueous sodium hydroxide solution. The resulting aqueous solution is purified, and then sulfuric acid and hydrogen peroxide used as an oxidizing agent are dropwisely added thereto to effect neutralization and oxidation of the dialkyldithiocarbamic acid. The resulting precipitate of the end product is subjected to filtration, washing with water, dehydration, drying, and grinding if so desired.
In the oxidative dimerization of sodium dialkyldithiocarbamates, oxidizing agents such as nitrogen dioxide (NO.sub.2), chlorine (Cl.sub.2), iodine (I.sub.2), ozone (O.sub.3), oxygen (O.sub.2), sodium nitrite (NaNO.sub.2), sodium hypochlorite (NaOCl), sulfur monochloride (S.sub.2 Cl.sub.2), sulfur dichloride (SCl.sub.2), potassium perbromate (KBrO.sub.3), selenic acid (H.sub.2 SeO.sub.3) and ammonium persulfate [(NH.sub.4).sub.2 S.sub.2 O.sub.8 ] can be employed instead of hydrogen peroxide. It should be noted that in those oxidative processes in which oxidizing agents are used, the stoichiometric quantities of oxidizing agents, neutralizing agents and the like are required, and special handling care must be taken as to the reaction apparatus and accessory apparatus as well as the process controls of the reaction.
On the other hand, a process for preparing tetraalkylthiuram disulfides via direct electrolytic oxidation of the corresponding alkali metal dialkyldithiocarbamates, in which use of the oxidizing agents and the like is avoided, is disclosed in U.S. Pat. No. 2,385,410. In this electrolytic oxidation process, the electrolytic reaction is carried out at a comparatively high temperature of 50.degree. to 60.degree.. It is well known that tetraalkylthiuram disulfides are generally inferior in thermal stability. The resulting disulfide could be partially decomposed thermally at a reaction temperature of 50.degree. to 60.degree. C., and the yield and quality of the product could thus be lowered.